Phase-Space Manipulation of Ultracold Ion Bunches with Time-Dependent Fields

All applications of high brightness ion beams depend on the possibility to precisely manipulate the trajectories of the ions or, more generally, to control their phase-space distribution. We show that the combination of a laser-cooled ion source and time-dependent acceleration fields gives new possibilities to perform precise phase-space control. We demonstrate reduction of the longitudinal energy spread and realization of a lens with control over its focal length and sign, as well as the sign of the spherical aberrations. This creates new possibilities to correct for the spherical and chromatic aberrations which are presently limiting the spatial resolution.

Figure 1

Schematic overview of the experiment. A laser-cooled cloud of rubidium (a) is trapped inside an accelerator structure (b). After pulsed ionization (c), ions are accelerated to the detector (d) where the transverse spatial distribution (e) and the temporal distribution are measured (f).

Figure 2

Illustration of the field terms in Eqs. (1a, 1b) for various anode voltage functions $V_a(t)$.

Figure 3

Results of measurement with a unipolar voltage pulse. The beam energy $U$ is varied by changing the pulse duration $\tau$ from 100 ns up to $2\mu \mathrm{s}$ while keeping $V_p$ constant. Three curves (black, red, and blue squares) with, respectively, $V_p=1000$, 500, and 250 V are shown. In (a) the longitudinal relative energy spread ${\sigma }_U/U$ versus $U$ is plotted and in (b) the transverse size on the detector ${\sigma }_x$ in the $x$ direction versus $U$. Additionally, a dc measurement (purple circles) is presented where $V_p$ is varied. Particle tracking simulations are depicted as solid curves (unipolar pulses) and a dashed curve (dc).

Figure 4

Demonstration of focusing by bipolar voltage pulses with $V_p=1000\mathrm{V}$. The transverse size in the $x$ direction ${\sigma }_x$ of the bunch on the detector is measured as a function of $V_n$. Phosphor images of the beam are shown at the bottom. The solid curve corresponds to particle tracking simulations.

Figure 5

Demonstration of the sign reversal of the spherical aberration coefficients. The position $x$ of the ion bunch on the detector is recorded as a function of the initial ionization volume position $x_0$ (scatter plots). In all measurements $V_p=1000\mathrm{V}$. The curves are fitted with the relation $x=A{x}_0-C{x}_0^3$ (solid curves). Results are shown of measurements using (a) a bipolar pulse ($C<0$) and (b) a tripolar pulse ($C>0$).